| 英文摘要： |
Using a morphometric dating method and the S-A (channel slope vs. drainage area) power law relation, the history of tectonic uplift and valley downcutting was investigated in the catchments of the Ten Kongduis (ten ephemeral streams) in Inner Mongolia, China, for the past ca. 5 Ma. These streams drain an area of the northern Ordos Plateau and flow northward into the Hetao Basin where they eventually debouch into the Yellow River. To derive reasonable ages of tectonic perturbation in catchments with obviously different widths between their lower and upper sections, an index was constructed for removing the effect of geometric irregularity on the morphometric ages. The morphometric analysis discloses an episode of tectonic uplift in the study area as early as 2.2-5.8 Ma, an episode or episodes of tectonic uplift after 1.15-0.50 Ma, and an episode of base level fall starting from 0.025-0.036 Ma. In the period of 1.15-0.50 Ma, the eastern stream catchments were uplifted from 0.99-1.15 Ma, a middle stream catchment was uplifted after 0.74 Ma, and a western stream catchment was uplifted after 0.50 Ma. The episode of base level fall after 0.025-0.036 Ma was detected only in some western streams, presumably indicating a recent subsidence episode of the western part of the Hetao Basin. After the episode of tectonic uplift dated to 2.2-5.9 Ma, equilibrium longitudinal profiles were formed in the streams and preserved in the sources of some streams. By comparing the elevations of the projected equilibrium profiles based on their S-A power law relations with the current ones, and considering the valley downcutting of the equilibrium profiles in the past 0.5-1.15 Ma, the largest valley downcutting magnitude upstream of the southernmost fault between the Ordos Plateau and the Hetao Basin in two western and three eastern streams was found to be in the ranges of 142-194 m and 52-145 m, respectively, and the base level fall at the northernmost fault between the plateau and the basin was estimated to be 317 m and 138 m on average for the two western and three eastern streams, respectively. (C) 2018 Elsevier B.V. All rights reserved. |
